1. Field of the Invention
The invention relates to projection displays, and more particularly to an improved contrast polymer dispersed liquid crystal display projection system.
2. Description of the Related Art
The cathode ray tube (CRT) has for many years been the cornerstone of computer and television display technology. It is not without its drawbacks. Larger sizes necessarily require larger vacuum tubes, more powerful magnets, and other components which increase in cost at a greater rate than the size of the display. It is not possible to make an arbitrarily large CRT display.
Because the display now represents a significant percentage of the cost of computer systems, and because the need for larger displays has increased with the graphical user interfaces that now prevail, alternatives to the CRT are very desirable.
A number of such alternatives have been developed. One technology is the liquid crystal display (LCD), which when used in a flat panel system presents a number of advantages over CRTs. But again, LCDs cannot be made arbitrarily large because of manufacturing limitations.
One promising technology involves projection displays. Projection televisions are well known, but have typically required three projection engines, one for each color. They do present the advantage, however, that an arbitrarily large surface can be used as the display surface, dependent on the intensity of the light source and optics of the projection system. That is, for large screens, projection display systems present unique advantages.
One type of projection system that has been developed employs polymer dispersed liquid crystals (PDLCs). Such liquid crystals have a polymer matrix that includes bubbles of liquid crystal. If no field is applied, the liquid crystal bubbles take on many different orientations. Because at least one of the indices of refraction of the liquid crystal differs from the index of refraction of the polymer, incoming light will be scattered.
If an electric field is applied, the liquid crystal bubbles will instead align with the field. If the materials are carefully controlled so the index of refraction of the liquid crystal for light polarized perpendicular to the electric field direction matches the index refraction of the polymer, then light will propagate through the material without being reflected and scattered by the bubbles.
A PDLC is typically provided with a reflective backing, and is used to reflect an intense light source when energized. Such a system is illustrated in FIG. 1. As is discussed below, in such a system, a light source is collimated onto the PDLC, which when fully energized passes the collimated light. The passed light is reflected by the backing, refocused through a pinhole aperture, and transmitted to the display surface. When the PDLC is not energized, the collimated light is instead scattered, so that very little light is reflected and refocused through the pinhole aperture onto the display. PDLC display systems are generally disclosed and discussed in U.S. Pat. No. 5,404,171 to Nague, et al., issued Apr. 4, 1995, which is hereby incorporated by reference. However, PDLC-based projection systems have a relatively low contrast ratio, at least because some of the scattered light will be transmitted through the pinhole aperture. Improvement of the contrast ratio of PDLC-based projection systems is desirable.